This invention relates to a device for shutting off the current through a start-up circuit for a motor using a thermistor with a positive temperature characteristic (herein referred to as the PTC thermistor) after a specified wait period has elapsed. This invention relates in particular to such a device adapted to be used in combination with such a motor start-up circuit.
FIG. 16 shows a prior art driving circuit for a motor 11, such as a single-phase inductive motor which may be used in a compressor for a refrigerator, comprising an auxiliary coil 12 which is activated at the start-up time of the motor 11 and a main coil 13 for a steady-state operation of the motor 11. The start-up circuit incorporated in this motor-driving circuit is for a motor of the so-called CSR (Capacitor Start and Run) type and is provided with a PTC thermistor 14 for motor start-up which is connected in series with the auxiliary coil 12.
A power source 16 is connected to the motor 11 through a switch 15. When the switch 15 is closed to supply power to the motor 11, a relatively large current flows in the beginning to the auxiliary coil 12 through the PTC thermistor 14 to start up the motor 11. A specified wait period of time after the start-up of the motor 11 has been completed, the PTC thermistor 14 serves to reduce the current to the auxiliary coil 12 by increasing its own resistance with the heat it generates.
There is a capacitor 17 for motor start-up connected in series with the PTC thermistor 14 and another capacitor 18 for the operation of the motor connected in parallel with the PTC thermistor 14 and the start-up capacitor 17. The start-up capacitor 17 is provided, if the motor 11 is a single-phase inductive kind, for providing a phase shift of 90 degrees for increasing the start-up torque given by the auxiliary coil 12. The motor-operating capacitor 18 serves to prevent pulsations after the motor 11 has been started up and to increase efficiency of its rotary motion. Either or both of these capacitors 17 and 18 may be unnecessary, depending on the kind of motor to be started up.
The portion of the circuit of FIG. 16, including the PTC thermistor 14 and enclosed by broken line 19, is commercially available as a component for the start-up of a motor. Five terminals 1, 2, 3, 5 and 6 for external connections are provided, and the PTC thermistor is encapsulated in a case (not shown) which supports these terminals 1, 2, 3, 5 and 6.
Since the resistance of the PTC thermistor 14 incorporated in the motor start-up component 19 of FIG. 16 does not become infinitely large, a wasteful current will continue to flow through it to the auxiliary coil 12 even after the motor 11 has been started up. The wasted power may reach the magnitude of several watts, and the PTC thermistor will continue to generate heat.
Japanese Patent Publication Tokkai 6-339291, on the other hand, disclosed a motor driving circuit incorporating a start-up circuit as shown in FIG. 17, wherein like components are indicated by the same numerals and may not be repetitiously explained. The circuit shown in FIG. 17 is characterized as having a triac 20, as well as a PTC thermistor 14 for motor start-up, connected in series with the auxiliary coil 12. There is also provided another PTC thermistor 21 for controlling the triac 20 connected in parallel with the motor start-up thermistor 14, one of the electrodes of the triac-controlling PTC thermistor 21 being connected to the gate G of the triac 20.
At the time of motor start-up with a circuit as shown in FIG. 17, when power from the source 16 is supplied to the motor 11, a trigger signal is applied to the gate G of the triac 20 through the triac-controlling PTC thermistor 21. This makes the triac 20 conductive and a current for the motor start-up is caused to flow to the auxiliary coil 12 through the motor start-up PTC thermistor 14. A specified length of time after the motor 11 has thus been started up, the motor start-up PTC thermistor 14 increases its resistance due to the heat it generates and thereby reduces the current to the auxiliary coil 12. At the same time, the triac-controlling PTC thermistor 21 reduces the current to the gate G of the triac 20 by increasing its own resistance due to the heat it generates. The triac 20 then returns to its OFF condition, shutting off the current to the auxiliary coil 12 no longer needed after the motor 11 has been started up.
A small current will thereafter continue to flow through the triac-controlling PTC thermistor 21 but since a thermistor with much smaller thermal capacitor can be used as the triac-controlling PTC thermistor 21 than as the motor start-up PTC thermistor 14, only a very small current is needed to keep the triac-controlling PTC thermistor 21 in a high-temperature condition with a high resistance. In summary, the power consumption can be significantly less than if the circuit shown in FIG. 16 is used.
The portion of the circuit of FIG. 17, including the triac 20 and the two PTC thermistors 14 and 21 and enclosed by the broken line 22 can also be unitized as a component for motor start-up, encapsulated in a case and provided with five terminals 1, 2, 3, 5 and 6 for external connections.
The current for starting up the motor 11, however, must be maintained for a specified length of time until the start-up is completed. The time it takes to become about one-half of the rush current is referred to as its operating time. The operating time becomes shorter as the volume of the motor start-up PTC thermistor 14 is reduced because its heat-up time also becomes shorter. Thus, motor start-up PTC thermistors having different volumes are necessary, corresponding to motors of different kinds, whether for the component 19 shown in FIG. 16 or for the component 22 shown in FIG. 17. For a motor requiring a long operating time, in particular, a large motor start-up PTC thermistor is required.
Let us consider next the return time, which is the time required after the motor 11 has started up and the current to the motor start-up PTC thermistor 14 has been shut off until the switch 15 is opened and can then be turned on again. There is no problem in turning on the switch a sufficiently long time after the current to the motor start-up PTC thermistor 14 has been shut off. When, for example, the component 22 is being used in connection with a motor for the compressor of a refrigerator, if the refrigerator door is opened and its inner temperature rises immediately after the thermostat has been switched off or if there is a momentary power failure and the motor 11 has been stopped such that the motor 11 must be restarted, a large motor start-up PTC thermistor 14 may not be quick enough to cool down for the start-up of the motor.
It has therefore been considered necessary to design the motor 11 by taking into account the time required for the motor start-up PTC thermistor 14 to lower its temperature. There are situations, however, where the design of the motor 11 cannot be easily modified. In such a situation, it is necessary to somehow improve the thermal radiation of the motor start-up PTC thermistor 14. As a practical matter, however, there is a limit to how much thermal radiation of the motor start-up PTC thermistor 14 can be improved.
Moreover, the size of the motor start-up PTC thermistor 14 is changed according to the operating time of the motor 11. As a result, the components 19 and 22 are required to be able to incorporate motor start-up PTC thermistors 14 of different sizes. If cases of only one size are prepared for these components 19 and 22, this size must be selected so as to be able to contain the largest of the motor start-up PTC thermistors 14. If a smaller one of the motor start-up PTC thermistors 14 is contained in such a case, not only does it give rise to a wasted space inside the case but these is a waste in terms of the material for the case. Although cases of a single size may be used for containing thermistors of different sizes, furthermore, other components for the electrical connections of elements therein such as the motor start-up PTC thermistors cannot be produced all in the same size, and components of different sizes must be prepared. This also contributes to a rise in the production cost. Thus, using cases of only one size cannot bring about a significant overall reduction in the production cost.
If cases of different sizes are to be prepared corresponding to the different sizes of the motor start-up PTC thermistors 14, on the other hand, the waste in the material for the cases can be reduced but the cost of the cases will go up caused by the production of more than one kind of products. In addition, many kinds of components such as terminals for electrical connections are required and their cost is also increased, and the number of different types of these components is also greater than if all cases are of the same size. As a result, the production efficiency is lowered, and the overall production cost will be higher than if cases of only one kind are to be produced.
It is also to be remembered that there are many ways to start up a motor. Thus, the components 19 and 22 require different kinds of terminals corresponding to different kinds of motor start-up. This is another factor to be taken into consideration.
The problems discussed above all apply both to the component 19 shown in FIG. 16 and the component 22 shown in FIG. 17 but they are more serious with the component 22 of FIG. 17. Suppose, for example, that it is being attempted to improve the thermal radiation of the motor start-up PTC thermistor 14 in order to reduce the time in which the motor 11 can be restarted. In the case of the component 22 shown in FIG. 17, however, there are both the triac 20 and the triac-controlling PTC thermistor 21 as sources of heat in addition to the motor start-up PTC thermistor 14, preventing thermal radiation of the latter. Moreover, the component 22 of FIG. 17 includes more constituent parts than the component 19 of FIG. 16 and hence requires a larger case. This is why the problems discussed above are more severe with the component 22 of FIG. 17. It is also to be noted that a material such as a heat-resistant resin must be used adequate for the motor start-up PTC thermistor 14 which emits more heat and reaches a high temperature than the triac-controlling PTC thermistor 21, and this affects the price of the cases. This means that it is disadvantageous to increase the size of the case.
Although not shown in FIG. 17, it is usually necessary to provide a metallic heat-radiating plate or the like to the heat-radiating part of the triac 20 in order to control the rise in its temperature 20 and its thermal run-away due to the motor start-up PTC thermistor 14. This is another reason that the component 19 of FIG. 17 has an increased number of parts, affecting the production cost adversely.